Molybdenum is a micronutrient, which is required by plants in very small amounts. At present molybdenum fertilisation is done using soluble salts like ammonium molybdate or sodium molybdate (J. J. Mortvedt, P. M. Giordano & W. L. Lindsay, 1972, Micronutrients in Agriculture, Soil Sci. Soc. Am., Madison). The disadvantage of using such soluble salts is that fertilizer use-efficiency is very low owing to leaching losses, fixation by minerals in the soil, etc. Water insoluble, slow-release forms of molybdenum fertilizers are practically unknown. Phosphate glass frits have been prepared which contain molybdenum in addition to other micronutrients (G. J. Roberts, 1977, Natl. Glass Budget. July 21). Such frits are usually prepared by fusing ammonium or sodium dihydrogen orthophosphates with micronutrient salts, at temperatures between 800° and 1400° C., to produce a melt and then rapidly cooling it by pouring on to a cold plate (G. J. Roberts 1973, Am. Ceram. Soc. Bull. Vol 52, p 383; ibid, idem, Vol 54, p 1069; Austrian Patent No 326160 of 1975; U.S. Pat. No. 3,574,591 of 1971; U.S. Pat. No. 2,713,536 of 1974; U.S. Pat. No. 3,958,973 of 1976). A Russian Patent (SU 1270148 of 1986) describes the production of a mixed metaphosphate based fertilizer containing Zn, Cu, Mo, Mn, Co, K, Na, etc. An Indian patent (No 175597 of 1991) describes the synthesis of a boron fertilizer, which is a slow-release compound.
The major disadvantage of glass frits is their process of production, which involves very high temperatures and highly corrosive melts. Another disadvantage is that, in such frits, nutrient release is controlled by hydrolysis of the compound and is dependent on soil pH, moisture conditions, etc. Therefore, rates of nutrient release may not always match crop requirements and the frits may not function as an effective nutrient source.
Another type of slow-release fertilizer has been produced in which the micronutrient ions are in a chemical form wherein they are insoluble but also plant available. These belong to the bio-release types of slow-release fertilizers. The processes for producing such phosphate based zinc and copper fertilizers are described in two Indian patents (Nos 172800 of 1990 and 177205 of 1991). The chemistry of zinc and copper phosphate polymerisation and the chemical nature of these fertilizers have also been described (S K Ray, C Varadachari & K Ghosh, 1993, Ind. Eng. Chem. Res. Vol. 32, p. 1218; S K Ray, C Varadachari & K Ghosh, 1997, J. Agric. Food Chem., vol. 45, p. 1447). Another Indian patent (Application No 10/Cal/99) describes processes for producing slow-releasing fertilizers of the cationic micronutrients.
The present invention provides processes for producing slow-releasing, water insoluble molybdenum fertilizers, which will also show good bio-availability of the nutrients and thus provide an effective source of molybdenum for plants. It also provides magnesium and phosphorous as additional plant nutrients.